stuck in fullscreen, host key doesn't work

Description

Linux host, XP guest, VBox 1.5.6. Been using Seamless, worked nice, one day I resume the VM (nothing changed that I knew of) and it goes fullscreen and stays there no matter how often I press anything. Lowering resolution of the guest only gives me a smaller area with black borders, display stays in host's native resolution. Only way to get my system back is ssh in and stop the VM with VBoxManager.